1. Field of the Invention
The present invention relates to 3-wire measurements. In particular, the present invention relates to remote 3-wire resistive measurements.
2. Discussion of the Related Art
Resistance temperature detectors (RTDs) and other resistive measurement elements are often required at remote locations. When making these resistive measurements, the lead wires to the resistive elements can contribute significant resistance and thus degrade the measurement accuracy. One method for reducing the effect of such resistance uses a 4-wire measurement, which requires four wires connected to the remote sensor. Many users have found such a measurement device undesirably complex.
One technique for reducing the effect of lead wire resistance is the “3-wire measurement.” A 3-wire measurement may reduce the effect of lead wire resistance, while only requiring three wires to connect to the remote sensor. FIG. 1 illustrates a typical configuration for a 3-wire measurement. As shown in FIG. 1, resistor RM, representing the resistance to be measured, is connected to a measurement device across terminals 101 and 102 by resistors RL1, RL2, Rp1 and Rp2. Resistors RL1 and RL2 represent the resistances of the lead wires, and resistors RP1 and RP2 represent additional resistances (e.g., resistances of protection resistors, which protect resistor RM from being shorted to a power supply). By measuring voltage VRM across resistor RM and the voltage across known current-sensing resistor RS between terminals 103 and 104, the resistance of resistor RM may be determined based on the known currents supplied by current sources 105 and 106. Terminal 104 is typically connected to a common ground reference. Ideally, voltage VRM across resistor RM is measured directly. However, when resistor RM is located at a remote location, a user has access only to voltage VM across terminals 101 and 102 of the measurement device. Ideally, VM=VRM. To ensure VM=VRM, currents I1 and I2 are forced by current sources 105 and 106 into leads connecting RM. For matching connections (i.e., RL1=RL2 and Rp1=Rp2), the voltage drops across resistors RL1 and Rp1 and across RL2 and Rp2 are the same, and thus VM=VRM. However, a mismatch in the connections results in errors. For example, when current sources 105 and 106 provide the same current (i.e., 11=12=I):VM=VRM+I*(RL1−RL2+RP1−RP2)Thus, the error between measured voltage VM and actual voltage VRM across resistor RM is proportional to the difference between the resistances in the two paths. With identical current sources, the average lead resistance between the two legs cancels each other, but the difference in lead resistance between the two legs does not. Therefore, the 3-wire measurement is accurate only to the extent that the lead resistances of the two legs can be minimized or equalized. Because wires have relatively low resistances and can be relatively easy to match, the 3-wire measurement technique is widely used.
However, the 3-wire measurement loses accuracy when additional resistors are added. Such additional resistors are added, for example, when an undesired shorting of resistor RM to a voltage source is possible. The additional resistors protect the measurement device from being damaged. The protection resistance (i.e., Rp1 and Rp2) can be significantly larger than the lead wire resistance (i.e., RL1 and RL2) to properly protect the measurement device. Often, a very significant mismatch exists between the resistances in the two legs that connect resistor RM. Because such a mismatch adversely impacts measurement accuracy, a 3-wire measurement that works effectively under such a condition is desired.
Two methods improve matching by either reducing the resistance in both legs or improving matching between the additional resistors of the two legs. Reducing the resistance in both legs results in a larger current being allowed to flow in the measurement device, thus decreasing input protection, increasing power dissipation (i.e., given by VM2/R, where R is the sum of RM, RL1, RL2, Rp1 and Rp2), and increasing circuit size and cost. Improving the matching between the additional resistors necessitates increasing cost. Secondary protection, such as a Zener diode, can be inserted between the resistive paths to provide a current sink at higher voltages. Such a diode adds cost and increases leakage currents, which can further degrade measurement accuracy. In general, the prior art systems suffer from difficult tradeoffs in cost, protection and accuracy.